The present invention relates to GaAs single crystals of high structural perfection and of high purity.
More particularly, the invention relates to GaAs single crystals with low dislocation density and low impurity content through contamination, diameter .gtoreq.1" with minimum variations in diameter of .+-.2 mm, 0.2 to 1 Kg weight.
It is known that the GaAs is a semiconductor used as substrate in devices, which find application in:
traditional telecommunication systems (radio and telephone links, T.V. etc.), PA1 communication systems on optic fiber (discrete and integrated components), PA1 distribution systems for television programs via satellite, PA1 radar systems for various uses, PA1 systems for switching high-speed numeric signals. PA1 Czochralski liquid encapsulation technique (LEC) and PA1 Bridgman horizontal oven technique (HBG).
Among the most well known GaAs devices, there are those optoelectric devices (photodetectors, senders, laser, light omitting diodes, etc.), and microwave devices (e.g. Gunn diodes, IMPATT diodes, Schottky diodes, etc.).
Moreover, GaAs is used in the integrated circuits: analogic, digital, monolithic integrated circuits of medium or large scale integration.
It is known from technical and patent literature, that the most used processes for the growth of GaAs single crystals on industrial scale are substantially two:
This latter technique has the advantage of giving low dislocation density crystals, but presents the drawback of giving a high silicon contamination, derived from the quartz tube used in the growth. Such contamination is intrinsic to this technology. Moreover, this technique is not very flexible, because it only allows one to obtain oriented crystals &lt;111&gt; with good characteristics and cylindrical shape but not with a circular section. This is a significant drawback because most of the electronic device use &lt;100&gt; oriented single crystals with cylindrical shape and circular section. Moreover the presence of high concentration of silicon does not allow the fabrication of devices which use semi-insulating GaAs, such as, for instance, microwave devices.
The Bridgman technique renders the scale passage to single crytals with high diameter and weight, problematic due to intrinsic difficulties of the technique itself.
Generally, in order to produce crystals with high diameter, usually .gtoreq.1" and varying weight, also exceeding 1 Kg, the LEC process is used. This method allows one to obtain crystals having the desired weight and dimensions (diameter) and circular sectioncylindrical shape, which obviously require growth times varying according to the weight. In this way, the presence of impurities is avoided and relatively reduced cycle times are used. However, the LEC technique, notwithstanding the various weight and diameter control methods usually used, presents the drawback of giving high dislocation density crystals, about 10.sup.4 -10.sup.5 Cm.sup.-2, and a high lack of structural and/or composition homogeneity both axial and radial, when the crystals have a diameter .gtoreq.1".
Moreover, in the LEC technique, polycrystals obtained with the Bridgman process are often used for the growth, giving single crystals with a high silicon impurity concentration. To improve the purity of the single crystal, the same Czochralski (CZ) oven for the synthesis in situ of the polycrystal is used, starting from the elements, Ga and As, and for the following successive growth of the single crystal.
However, even single crystals which are grown with this expedient have a dislocation concentration which is too high, a lack of structural homogeneity due to the process of microdefects, microprecipitates, etc., and moreover, frequently present a lack of uniformity in diameter, generally .+-.3-5 mm for diameter .gtoreq.1". Moreover, these characteristics tend to become worse with the increase of weight. This lack of diameter homogeneity provokes remarkable loss of utilizable material in processing, in that this requires constant diameter slices.
Moreover, said lack of diameter homogeneity has an influence in the formation of reticular defects, particularly dislocations.
The high number of dislocations and the lack of structural homogeneity have negative effects in the processing of the wafers (slices) because of diffusive phenomena, and consequently have a negative influence on the characteristics of the devices, which are reproducibility, average life, noise, etc.
For instance, in the epitaxial growth processes, the use of wafers (slices) containing a high number of defects provokes migration of the defects and of the impurities from the substrate to the epitaxial layer, above all in the vicinity of the junction.
To diminish the dislocation density, the crystals are generally doped with dopants, such as for instance, boron, silicon, selenium, sulphur, tellurium, in quantities comprised between 10.sup.16 -5.times.10.sup.17 atoms/cm.sup.3.
In particular, it is observed that with some of these dopants the dislocations tend to disappear when using a higher concentration of dopant generally comprised between 5.times.10.sup.17 -10.sup.19 atoms/cm.sup.3.
Methods normally used to diminish the dislocations with dopants are described in the published U.K. patent application 2.108.404 A.
These systems for reducing the dislocations with the use of dopants are not suitable for preparing single crystals to be used as intrinsic semiinsulators, in that the presence of the dopants renders the single crystals semiconductors.
The resistivity .rho. in fact, lowers from the values of about 10.sup.7 .OMEGA. cm. to values &lt;10.OMEGA. cm.
The presence, for instance, of 10.sup.16 -10.sup.19 atoms/cm.sup.3 of dopant, renders the single crystals unsuitable for applications as intrinsic semiinsulators.
Only for certain applications, such as for instance, for solar cells, dopant quantities of 10.sup.17 -5.times.10.sup.18 atoms/cm.sup.3 are required.
Therefore, it was desirable to obtain GaAs single crystals with such a low dislocation density as to make them suitable for applications as intrinsic semiinsulators, for instance, for integrated logic, FET or GUNN diodes.
Moreover, a low number of dislocations makes the single crystals preferable in that it allows for high quality devices with good signal reproducibility, low noise, high average life.
However, it also desirable to have available, single crystals having a low number of dislocations also for certain applications wherein doped single crystals are required.
In fact, if the number of dislocations is high, it is necessary to use a high quantity of dopant, between 5.times.10.sup.17 and 10.sup.19, to reduce it.
This presents notable drawbacks in that microprecipitates can be generated which give devices with electric characteristics of poor quality, for instance, the cutoff frequency in the Gunn diodes is lowered, as the average life of laser, and the signal propagation velocities in electronic systems are lowered, such for instance, in telecommunications, in computers, in radar systems, etc.